Radiosurgery and radiotherapy systems are radiation therapy treatment systems that use external radiation beams to treat pathological anatomies (tumors, lesions, vascular malformations, nerve disorders, etc.) by delivering prescribed doses of radiation (X-rays, gamma rays, electrons, protons, and/or ions) to the pathological anatomy while minimizing radiation exposure to the surrounding tissue and critical anatomical structures. Radiotherapy is characterized by a low radiation dose per fraction (e.g., 100-200 centiGray), shorter fraction times (e.g., 10-30 minutes per treatment), and hyper fractionation (e.g., 30-45 fractions), and repeated treatments. Radiosurgery is characterized by a relatively high radiation dose per fraction (e.g., 500-2000 centiGray), extended treatment times per fraction (e.g., 30-60 minutes per treatment), and hypo-fractionation (e.g. 1-5 fractions or treatment days). Due to the high radiation dose delivered to the patient during radiosurgery, radiosurgery requires high spatial accuracy to ensure that the tumor or abnormality (i.e., the target) receives the prescribed dose while the surrounding normal tissue is spared.
In general, radiosurgery treatments consist of several phases. First, a precise three-dimensional (3D) map of the anatomical structures in the area of interest (head, body, etc.) is constructed using any one of (or combinations thereof) a computed tomography (CT), cone-beam CBCT, magnetic resonance imaging (MRI), positron emission tomography (PET), 3D rotational angiography (3DRA), or ultrasound techniques. This determines the exact coordinates of the target within the anatomical structure, namely, locates the tumor or abnormality within the body and defines its exact shape and size. Second, a motion path for the radiation beam is computed to deliver a dose distribution that the surgeon finds acceptable, taking into account a variety of medical constraints. During this phase, a team of specialists develop a treatment plan using special computer software to optimally irradiate the tumor and minimize dose to the surrounding normal tissue by designing beams of radiation to converge on the target area from different angles and planes. Third, the radiation treatment plan is executed. During this phase, the radiation dose is delivered to the patient according to the prescribed treatment plan.
To help deliver radiation therapy to the target according to the radiation treatment plan, image-guided radiation therapy (IGRT) can be used. IGRT is a radiation therapy process that uses cross-sectional images of a patient's internal anatomy taken during the radiation therapy treatment session (i.e., in-treatment images) to provide information about the patient's position. IGRT is thus a process of frequent two or three-dimensional imaging during the course of the radiation treatment used to direct the therapeutic radiation utilizing the imaging coordinates of the actual radiation treatment plan. This ensures that the patient is localized in the radiation treatment system in the same position as planned, and that the patient is properly aligned during the treatment. The IGRT imaging process is different from the imaging process used to delineate targets and organs in the planning process of radiation therapy (i.e., different from the pre-treatment images obtained during the first phase). Although, the IGRT process involves conformal radiation treatment guided by specialized imaging tests taken during the first phase, it does rely on the imaging modalities from the planning process as the reference coordinates for localizing the patient during treatment. Thus, associated with each image-guided radiation therapy system is an imaging system to provide in-treatment images that are used to set-up the radiation delivery procedure.
In-treatment images can include one or more two or three-dimensional images (typically X-ray) acquired at one or more different points of view during treatment. There are a variety of ways to acquire the in-treatment images. In certain approaches, distinct independent imaging systems and/or imaging methods are used for acquiring pre-treatment and in-treatment images, respectively. For example, a 3D IGRT could include localization of a cone-beam computed tomography (CBCT) dataset with a planning computed tomography (CT) dataset, and a 2D IGRT could include matching planar kilovoltage (kV) radiographs or megavoltage (MV) images with digital reconstructed radiographs (DRRs) obtained from the planning CT. Alternatively, another approach is to use portal imaging systems. In portal imaging systems, a detector is placed opposite the therapeutic radiation source to image the patient for setup and in-treatment images. Another approach is X-ray tomosynthesis which is an in-treatment imaging modality for use in conjunction with radiation treatment systems. X-ray tomosynthesis is a process of acquiring a number of two-dimensional X-ray projection images of a target volume using X-rays that are incident upon the target volume at respective number of different angles, followed by the mathematical processing of the two-dimensional X-ray projection images to yield a set of one or more tomosynthesis reconstruction images representative of one or more respective slices of the target volume.
In image-guided radiotherapies, there are many factors that can contribute to differences between the prescribed radiation dose distribution and the actual dose delivered (i.e., the actual dose delivered to the target during the radiation treatment). One such factor is uncertainty in the patient's position in the radiation therapy system. Other factors involve uncertainty that is introduced by changes that can occur during the course of the patient's treatment. Such changes can include random errors, such as small differences in a patient's setup position. Other sources are attributable to physiological changes that might occur if a patient's tumor regresses or if the patient loses weight during therapy. Another category of uncertainty includes motion. Motion can potentially overlap with either of the categories as some motion might be more random and unpredictable, whereas other motion can be more regular. These uncertainties can affect the quality of a patient's treatment and the actual radiation dose delivered to the target.
In existing radiosurgery therapy systems, establishing precision of therapeutic dose delivery is done by carefully calibrating the position and orientation of a well characterized imaging system with respect to the therapeutic radiation beam used during the treatment. The accuracy of the radiosurgery is, therefore, dependent on the fidelity with which this calibration process if performed.
In stereotactic radiosurgery (SRS), which is a highly precise form of radiation therapy used to treat tumors and abnormalities of the brain, 3D imaging, such as CT, MRI, and PET/CT, is used to generate pre-treatment images to locate the tumor or abnormality within the body and define its exact size and shape. SRS also relies on systems to immobilize and carefully position the patient's head during the initial imaging phase (pre-treatment imaging) as well as during the radiation therapy session. Therefore, in SRS, the pre-treatment images show the exact location of the tumor in relation to the head frame. In stereotactic radiosurgery, this calibration can utilize rigid external frame-based head immobilization which, when properly calibrated, reliably locates the head or other body parts in a known position with respect to the therapeutic beams. However, if the calibration is done incorrectly or if post pre-treatment imaging the head slips inside the external frame or the radiosurgery instrument is knocked out of alignment after calibration, a user may not know, and serious consequences are possible.
In the existing radiation therapies, therefore, radiation delivery is made based on the assumption that the radiation treatment plan was developed based on correct information, the position of the radiation beam relative to the patient set-up is correctly calibrated, and that the radiation therapy system not only functions properly but that it also functions based on correct and consistent external inputs used to program the system. However, if the calibration of the support device, for example, is incorrect, or the radiation therapy system functions improperly, or the treatment plan may include incorrect information, an incorrect dose will be delivered to the target during treatment even if the radiation therapy system operates as instructed.
Also, radiation therapy detectors typically are either not designed to detect high energy therapeutic beams or not designed to be constantly operating during radiosurgery, and therefore real-time validation of the target treatment volume is not feasible.